Inhibition of oxidation in tin solutions



bath by periodic additions.

Patented Feb. .19, 1952 UNITED STATES PATENT OFFICE INHIBITION OF OXIDATION IN TIN SOLUTIONS- Allen G. Gray, Niagara Falls, N. Y., assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Application February 2, 1949, Serial No. 74,260

This invention relates to the electroplating of tin and more particularly to the electroplating of tin from aqueous electrolytes containing stannous chloride and alkali metal fluoride,

Solutions of stannous chloridecontaining relaprocess, described in Schweikher, U. S. P.-

2,407,579. This process utilizes, as electrolyte, an acidic stannous chloridesolution containing 2 to 12 molesof alkali metal fluoride per mole of stannous chloride.

'In operating tin plating baths of this character, considerable difflculty is caused by the formation of insoluble material, chiefly sodium fluo-stannate, which settles to the bottom of the plating bath, forming a sludge which must be periodically removed. This sludge formation appears to be caused by oxidation of the stannous chloride due to the contact of the electrolyte with the air. Generally the accumulation .of sludge in the plating equipment is such that after sev eral months operation the process must be shut down to remove the sludge, a time-consuming and expensive manual operation. amounts of sludge formed also represent considerable monetary loss because of the tin content, which can be off-set only by processing the sludge to recover the tin in a saleable form.

Various means have been proposed for inhibiting the oxidation of the stannous chloride and hence the sludge formation. The most satisfactory method known heretofore is to add to the electroplating bath a small amount of an alkali metal ferrocyanide or ferricyanide. In a typical operation, utilizing the process of the above Schweikher patent, a concentration of 05 to 1.5 grams per liter of sodium ferrocyanide, expressed as the decahyd-rate, is maintained in the plating While this greatly reduces the sludge formation, nevertheless, in extended periods of operation undesirably large amounts of sludge are formed and means to further decrease sludge formation have been sought.

,An object of the present invention is to inhibit the air oxidation of aqueous solutions of stannous chloride containing :alkali metal fluoride. A further object is to inhibit the formation of the sludge or other insoluble matter in such solutions when theyare-utilized as tin plating baths in con- The large I 5 Claims. (Cl. 204-54) 2 tact with the air. A still further object is to enhance the action of alkali metal ferrocyanides and ferricyanides in inhibiting the aforesaid oxidation or sludge formation. Still other objects will be apparent from the following description of the invention. I

The above objects may be attained-by adding to the stannous chloride-alkali metal fluoride solution a small amount of thiourea, preferably in conjunction with the addition of an alkali metal ferrocyanide or ferricyanide. In utilizing such solutions as tin plating baths, preferably, a concentration of, about 1 to 5 grams per liter of thiourea is maintained in the plating solution.

Although it is known that the presence of thiourea accelerates the rate of oxidation of acidic stannous chloride solutions (Haring and Walton, J. Phys. Chem. 37, 133-145 (1933)) I have discovered that in an acidicstannous chloride solu-' tion which contains from 2 to 12 moles of an alkali metal fluoride per mole of stannous chloride, thiourea has the opposite effect and markedly decreases the rate of oxidation of the stannous chloride when the solution is exposed to the air.

I havefurther discovered that this oxidation-decreasing effect of thiourea is most marked when an alkali metal ferrocyanide or ferricyanide has also been added to the bath as oxidation inhibitor. Further, I have found by comparative tests that the oxidation inhibiting effect obtained by adding both an alkali metal ferrocyanide and thioureato such solution is considerably greater than the effect obtained with either the ferrocyanide or thiourea alone.

The following examples illustrate the practice of my invention:

' Example I A standard tin plating solution was prepared having the following composition:

The polyethylene oxide is an addition agent commonly added to tin plating baths of this character to improve the quality of the electrodeposit.

To simulate conditions known to occur in commercial tinplating baths, ferric sulfate and copper sulfate (CuSO4) were added to the above solution in amounts equivalent to 0.05 g./l. of Fe+ and 0.005 g./l. of Cu++.

A cc. portion of the solution was shaken for two hours in a 250 cc. flask, mounted in a Burrell shaking machine, while oxygen was admitted to the flask under a pressure of lbs. per sq. in. By titrating samples of the solution with a standard KIOa solution before and after the shaking operation, the amount of oxidation was determined and expressed as the percentage of the stannous tin oxidized to the stannic state.

The operation then was repeated with 50 cc. portions to which had been added oxidation in.- hibitors as noted in the table below; The following results were obtained:

- Inhibitcr(s) Con- Per Cent Test Inhlbltofls) centration(s) Oxidation A..." None 66. B Sodium ferrocyanide 0.75 g. 40. C. Thicurea l0g./l. 48. D Sodium ferrocyanide plus 0.75 g./l 2 Thiourea. 1.0 g./l.. En Sodium ferrocyanideplus {0.75 g./l 11 w Thiourea. 1.0 g./l. i

Example 11 From various parts of acom mercial tin plating process in operation, samples of the plating bath were taken'and to a portion of each sample 1 g./l. of thiourea was added: The plating bath' was made up according to the formula given abovein Example I and each samplecollected contained sodium ferrocyanide as shown bya conventional qualitative test; Portions ofthe samples, with and without the added thiourea were subjected to a two-hour oxidation test by the method'of ExampleIII A quantity,( 15 gals.) of the standardjin plat-T ing solution of Example I was continuously aerated in an apparatus consisting of a reservoir, a pump, an aeration chamber and a fiowmeas'ur l ing device, all connected by. suitable -p'iping' as hereinafter apparent. The solution,'placed in the reservoir was pumped to the bottom; of .the'aeration chamber, which was an open tank provided with an overflow outlet position 8-inches ab'cveas bottom. A valved opening onithe suction side of the pump admitted a controlled flow of airinto the pump, where the airmixed with the solution passing through. The resulting air-solution mixt'urefipassing from the pump was admitted to the aeration'cha'mber by a horizontal, perforated pipe resting on the floor of the chamber. The aerated solution passed'out the overflow, through a flow measuring device and thence returned to thereservoir.

For aperiod of 6 days, the 15 galsyof solution was continuously pumped through the apparatus and thus continuously aerated. During this period; a dilute ferric sulfate solution was continuously fed into thereservoi'r by means of a proportioning pumpat a rate equivalent to 0.05 g./l. o'fFe' /24 hours. 'Also periodic additions of sodium ferrocyanide were made as required 'to maintain a"concentration of'0.5 to 0.75'gi/l. The

4 solution temperature in the aerator was maintained at 60 C.:l.

After 4 days operation thiourea was added in concentration of 2 g./l., while the aeration continued for two days longer.

The amount of oxidation during each 24 hour period was determined by periodic titration of samples. The following results were obtained:

DB Thiourea Content Per Cent y 01' Solution Oxidation Each day, a sample of the solution undergoing aeration was removed and used as electrolyte to electroplate tin onto steel cathodes. The electroplating conditions were:

Bath temperature, 60 C. .f I Current density, 50-60 amps. per sq. ft. Plating time, 30 seconds.

' The resulting electrodeposits were of standard commercial quality and those obtained after'the addition of thiourea were identical with those plated before addition of thiourea. i

In practicing the present invention, theacidity of the stannous chloride 'alkali metal fluoride solution should be maintained above a pH of about 2, since at'a lower pH the inhibiting effect of the thiourea becomes inappreciable. In electroplating operations I prefer to maintain the acidity of the plating bath'at pH 2.5 to 3.8, although a pH as high as 5 may beused'under conditions apparent "from 'the above-mentioned Schweikher patent.

In a preferred mode 'of practicing the present invention 'tin is electroplated from a stannous chloride solution as described: in the above Schweikher patent, 2,407,579, using-tin anodes. provided that the pH is above 2 but not greater than 5. The plating bath prepared in accordance with the Schweikher patent may contain from' 37.5 to grams per liter of stannous chloride, and 37.5 to 150 grams per liter of an alkali metal fluoride, provided that the molar concentrations of the stannous chloride and the fluoride and the pH of the plating bath are related as expressed by the following formula:-

mole's' SnCh where lVl'F' represents alkali metal fluoride and k has'a value between 0.1 to l; and provided also,

W V G./l.' Stannous chloride (SnClz'.2HzO) 60 Sodium fluoride (NaF) 30 Sodium difluoride (NaHFz) 30 Sodium chloride (NaCl) 40 In operating such a bath according to a preferred embodiment of my invention, I may add thereto alkali metal ferrocyanide or' ferricyanide' (generallysodium' ferrocyanide is preferred) in sufficient amount to maintain a concentration of about 0.5 to 1.5 grams per liter. I further add to the bath 1 to .5 grams .per liter of thiourea; and similar additions are made .from time .to time as may be necessary to maintain the-above stated concentrations. I also may. add one .or more :organic compounds as addition agents which are known to improve the quality of the electrodeposit but such addition is not an essential feature of the present invention.

As noted above, the inhibition of oxidation is greater when both thiourea and an alkali metal ferrocyanide or ferricyanide are added as inhibitors. However, to obtain this synergistic effect in appreciable degree, the solution should contain not less than 0.5 gram per liter of alkali metal ferrocyanide or ferricyanide; generally a concentration of 0.5 to 1.5 grams per liter is preferred. Any larger amounts may be added, as may be desired, with no deleterious effect, so far as oxidation inhibition is concerned. As a practical matter, it is seldom desirable to add more than grams per liter to a plating bath, as there is some decomposition.

The amount of ferrocyam'de or ferricyanide present may bedetermined by known analytical methods. For example, the presence of ferrocyanide can be quantitatively detected by adding ferric chloride to a sample of the solution and noting the blue color caused by the formation of ferric ferrocyanide. By comparing the color intensity with that produced in a series of samples of known ferrocyanide content, the ferrocyanide can be determined quantitatively.

While I generally prefer to add about 1 to 5 grams per liter of thiourea, good results can be obtained over the range of 0.1 to 10 grams per liter. Still larger quantities may be added if desired, but generally are no more effective than 5 to 10 grams per liter.

I have discovered that the addition of thiourea has no appreciable effect either adversely or advantageously on the quality of the electrodeposited tin. The only effect of the thiourea that can be ascertained is the effect of inhibiting the oxidation of stannous chloride and thereby reducing the amount of sludge formation.

My invention is not restricted to solutions consisting of stannous chloride and alkali metal fluorides, but is likewise useful in inhibiting tin oxidation in such solutions which additionally contain other ingredients, particularly ingredients commonly present in acidic tin plating baths. For

example, any of the alkali metal chlorides may be added to serve as conducting salts. Also various addition agents both inorganic and organic, such as those mentioned in the above Schweikher patent may be added to improve the quality of the electrodeposit. In other words, my invention includes the addition of thiourea to any stannous chloride solution which has a pH between 2 and 5 and contains one or more of the alkali metal fluorides (including the difiuorides, e. g., NaI-IF'z) in a proportion equivalent to 2 to 12 moles of the monofluoride (e. g., NaF) to one mole of stannous chloride. While I prefer to add the thiourea to such solutions which contain 37.5 to 150 grams per liter of stannous chloride, such being best adapted for tin plating, the invention is not restricted thereto. Regardless of the stannous chloride concentration, the addition of thiourea will inhibit oxidation of the stannous chloride so long as the solution has a pH between 2 and 5 and contains 2 to 12 moles of alkali metal fluoride'per mole of stannous chloride.

6 The presence of ions of heavy metals such as iron 'andccpper apparently catalyzes or promotes the oxidation of the stannous chloride, and the addition of thiourea is of especial advantage when such heavy metal ions are present.

The present invention affords a means for inhibiting oxidation of acidic stannous chloride solutions of the character described and is of especial advantage for decreasing sludge formation due to oxidation in such solutions in tin plating operations. When used in conjunction with alkali metal ferrocyanide or ferricyanide, the addition of thiourea generally reduces the amount of sludge formation to about half the quantityformed when the ferrocyanide or ferricyanide is used alone. The result is that the plating operation can be continued twice as long before it is necessary to shut down for sludge removal.

In electroplating tin in accordance with my invention, I prefer to electrolyze the solution with tin anodes and steel cathodes, but the invention is not restricted thereto.

I claim:

1. In the operation of a tin electroplating process wherein tin is electrodeposited from an aqueous, acidic solution of stannous chloride having a pH between 2 and 5 and containing 2 to 12 moles of alkali metal fluoride per mole of stannous chloride, the improvement which consists in adding thereto 9.1 to 5 grams per liter of thiourea.

2. In the operation of a tin electroplating process wherein tin is electrodeposited from an aqueous solution containing 37.5 to grams per liter of stannous chloride and 37.5 to 150 grams per liter of alkali fluoride in the proportions of 2 to 12 moles of said fluoride to one mole of stannous chloride and having a pH between 2 and 5 and the formation of sludge in said electrolyte is inhibited by adding thereto a compound of the group consisting of alkali metal ferrocyanides and ferricyanides, the improvement which consists in adding to said electrolyte about 1 to 5 grams per liter of thiourea.

3. In a tin electroplating process the step comprising electrodepositing tin from an aqueous bath comprising from about 37.5 to 150 grams per liter of an alkali fluoride, and from about 37.5 to 150 grams per liter of stannous chloride and satisfying the equation:

k(mo1es MF) moles SnCl wherein the following conditions are simultaneously true; the pH is between 2 and 5, It has a value "from 0.1 to 1.0, MF is alkali fluoride, and the mole ratio is about from 2 to 12, the static solution potential of tin in the bath being equal in volts to from 0.055 pH-0.265 to 0.055 pH-0.3'70 said bath also containing 0.5 to 1.5 grams per liter of sodium ferrocyanide and about, 1 to 5 grams per liter of thiourea.

4. The process which comprises electrodepositing tin from an aqueous, acidic solution of stannous chloride having a pH between 2 and 5 and containing 2 to 12 moles of alkali metal fluoride per mole of stannous chloride and about 1 to 5 grams per liter of thiourea.

5. The process which comprises electrodepositing tin from an aqueous, acidic solution of stan- REFERENCES CITED The following references are of record in the file of this patent:

" I UNITED STATES PATENTS :Number 1 Name Date 1,903,860 Gockel Apr. 18, 1933 2,369,620 Sullivan Feb. 13, 1945 5 2,402,185 Schweikher June 18, 1946 2,407,579 Schweikher Sept. 10, 1946 OTHER REFERENCES Haring et al., Journal of Physical Chemistry. volume 37 (1933), p 133-145. 

1. IN THE OPERATION OF A TIN ELECTROPLATING PROCESS WHEREIN TIN IS ELECTRODEPOSITED FROM AN AQUEOUS, ACIDIC SOLUTION OF STANNOUS CHLORIDE HAVING A PH BETWEEN 2 AND 5 AND CONTAINING 2 TO 12 MOLES OF ALKALI METAL FLUORIDE PER MOLE OF STANNOUS CHLORIDE, THE IMPROVEMENT WHICH CONSISTS IN ADDING THERETO 0.1 TO 5 GRAMS PER LITER OF THIOUREA. 